There are various designs for electronically commutated motors. One known classification system is based on the number of current pulses supplied to the stator of such a motor for each rotor revolution of 360° el. A distinction can therefore be made between one-pulse motors, in which only a single driving current pulse is supplied during one rotor revolution of 360° el.; two-pulse motors, in which two stator current pulses, which are usually spaced apart in time from one another, are supplied during one rotor revolution of 360° el.; and also three-pulse, six-pulse, etc. motors.
Such motors are further classified according to their number of stator winding strands, i.e. as one-strand, two-strand, three-strand motors, etc.
For complete definition of a design, the number of stator winding strands and the number of pulses per 360° el. must therefore be indicated, e.g. a two-pulse, two-strand motor. Borrowing from the terminology of motors that are operated with alternating or three-phase current, two-pulse motors are also referred to as single-phase motors; a single-phase motor can therefore have either one or two winding strands.
In a two-strand motor there is a first series circuit made up of a first winding strand and a first controllable semiconductor switch, and a second series circuit made up of a second winding strand and a second controllable semiconductor switch. Current is supplied alternately to the two winding strands in order to produce a magnetic field necessary for rotation of the permanent-magnet rotor. (In general, such a motor is also implemented to generate a so-called reluctance torque in the rotational position regions where the electrically generated torque has gaps; cf. for example DE 23 46 380 C2, Müller, corresponding to U.S. Pat. No. 4,374,347.)
A motor of this kind is usually operated from a direct current source, e.g. from a battery, a power supply, or a rectifier that rectifies the voltage of an alternating or three-phase power network and supplies it to a DC link circuit from which the motor is supplied with direct current. A capacitor, referred to as a link circuit capacitor, is usually connected to this link circuit.
When current flows through a winding strand, energy is stored in it in the form of a magnetic field. If the inductance in such a strand is designated L, and the current I, this energy can be calculated using the formulaW=0.5*L*I2  (1).
If a rotating magnetic field is to be generated by switching over from a first to a second winding strand (this being referred to as “commutation”), this stored energy must first be dissipated.
When a current-carrying winding strand is switched off, the effect of so-called self-induction at that winding strand is to cause a voltage rise that is brought about by the stored magnetic energy. Very high voltages can be caused as a result. Semiconductor switches having high dielectric strength must therefore be used.
A certain improvement can be achieved by using a link circuit capacitor, which serves to receive, in the form of electrical energy, the energy stored magnetically in the winding strand, and thereby to limit the voltage that occurs at the motor's DC link circuit. This capacitor therefore receives energy in operation and then immediately discharges it again; in other words, a current, also referred to as a “ripple current,” continuously flows in the leads of this capacitor. The larger the capacitor required, the greater the ripple current becomes.
In terms of material costs, capacitors of this kind represent an economical solution to the aforementioned problem, but relatively large capacitors—usually so-called electrolyte capacitors—are required; their service life is limited, and is additionally reduced by the considerable heating that unavoidably occurs during the soldering operation and because of the ripple current. This decrease in the service life of the capacitor therefore has an effect on that of the motor.
A further possibility for limiting the voltage spikes that occur when a winding strand is switched off is to use Zener diodes or, when a FET (Field Effect Transistor) power stage is utilized, to exploit the so-called avalanche energy. Here the energy, that is stored upon shutoff in the winding strand that is to be switched off, is converted into heat in the aforesaid semiconductor elements. From the viewpoint of the semiconductor elements that are used, this is dissipated power, and components of appropriate performance must therefore be used.
The energy converted into heat is also “lost” and can no longer be used to drive the rotor, i.e. the efficiency of such a motor is lower.